This invention relates to an apparatus for detecting the local temperature variations of an object and the sites of the temperature variations.
Recently, an apparatus for detecting local temperature variations of an object and the sites of the temperature variations (hereinafter referred to as "a temperature variation-site detecting apparatus") has been developed which detects abnormal temperatures occurring, for example, in an elongate pipe on a tank or the leakage of a hot or cold liquid and the sites of all these accidents by utilizing changes with time in the characteristic impedance of a temperature variation-detecting cable comprising a magnetic material disposed between a pair of conductors.
The U.S. Pat. No. 4,023,412 sets forth the above-mentioned type of temperature variation-site detecting apparatus. FIG. 1 is a schematic diagram of an example of a prior art temperature variation-site detecting apparatus. A temperature variation-detecting cable 32 is mounted on part of the outer peripheral wall of an object 30, for example, a tank. The cable 32 is, as shown in FIG. 2, of the concentric type which comprises a linear core conductor 34, outer hollow cylindrical conductor 36 surrounding the core conductor 34 and a magnetic material 38 which has a retatively low Curie temperature and is filled between the outer conductor 36 and core conductor 34. The magnetic material 38 is formed by mixing ferrite powder with, for example, an insulating adhesive. This magnetic material 38 is chosen to have such a Curie temperature as is set at the upper or lower limit of a temperature range regarded as, for example, an abnormal level.
The starting end of the cable 32 in connected to a pulse generator 40 and pulse time difference detector 42. The terminal end of the cable 32 is short-circuited or may be left open. Upon receipt of a set signal, the pulse generator 40 sends forth a short width pulse signal to the starting end of the cable 32. The pulse time difference detector 42 counts a difference between a point of time at which the output pulse signal is received and a point of time at which the pulse signal is reflected from the prescribed spot of the cable 32. A signal denoting the time difference is conducted to a detector 44 for indicating a site at which temperature variations take place (hereinafter referred to as "a temperature variation site detector). This temperature variation site detector 44 determines the occurrence or absence of abnormal temperature and the site of the abnormal temperature from the above-mentioned time difference thus detected.
A pulse signal emitted from the pulse generator 40 is supplied to the starting end of the cable 32, and then conducted through the cable 32. Where, at this time, a substantially fixed temperature prevails over the whole of the object 30, then the impedance of the cable 32 also remains fixed over the whole of the object 30. Therefore, the output pulse signal is reflected only at the terminal end of the cable 32. At this time, the pulse time difference detector 42 counts a difference T.sub.0 (T.sub.0 =2 l/v) (FIG. 3(a)) between the point of time at which an output pulse signal P.sub.1 is received and the point of time at which a reflected pulse signal P.sub.2 is received at the terminal end of the cable 32. The character l denotes the total length of the cable 32, and the character V represents the speed at which a pulse signal is transmitted through the cable. Where a time difference T.sub.0, for example, is detected, then the temperature variation site detector 44 judges that the object 30 has no abnormal temperature variations.
Where the temperature of the portion of the cable 32 near a given point A on the object 30 at which an abnormally high temperature occurs increases over the Curie temperature of the magnetic material 38, then the relative permeability of the magnetic material 38 substantially stands at 1. As a result, the impedance of the cable 32 at point Q.sub.1 near the aforementioned point A falls, causing the above-mentioned pulse signal to be reflected at the point Q.sub.1. At this time, the pulse time difference detector 42 counts a difference T.sub.1 (T.sub.1 =2 l.sub.1 /v) (FIG. 3(b)) between a point of time at which the output pulse signal P.sub.1 is received and a point of time at which a pulse signal P.sub.3 is reflected at point Q.sub.1. The character l.sub.1 is a distance from the starting end of the cable 32 to point Q.sub.1. Where the aforementioned abnormal temperature site A covers an area defined by a prescribed distance l.sub.2, then the pulse time difference detector 42 also counts a difference T.sub.2 (T.sub.2 =2 l.sub.2 /v) between a point of time at which the pulse P.sub.3 is reflected at point Q.sub.1 of the abnormal temperature site A and a point of time at which a pulse P.sub.4 is reflected at point Q.sub.2. Where the time differences T.sub.1, T.sub.2 (T.sub.1 &lt;T.sub.0, T.sub.2 &lt;T.sub.0) are detected, then the temperature variation site detector 44 judges that an abnormal temperature arises in the object 30, and determines the site and range of the abnormal temperature.
The aforementioned prior art temperature variation-site detecting apparatus indeed has the advantages that it is possible to easily detect the occurrence of abnormal temperatures in an object, for example, a tank, and the sites of the abnormal temperatures; and sites of abnormal temperatures can be detected over a broad area of the object 30, thereby making it possible to easily determine sites of abnormal temperatures even in a large object.
Nevertheless, the above-mentioned prior art temperature variation-site detecting apparatus is accompanied with the following drawbacks. The cable 32 is constructed by uniformly filling a magnetic material 38 in a space defined between the core conductor 34 and outer hollow cylindrical conductor 36. In this case, the conventional process comprises mixing ferrite powder with an adhesive, and filling the mixture between the conductors 34, 36 or applying ferrite powder on the outer peripheral wall of the core conductor 34 by means of a rubber type adhesive. Consequently this conventional apparatus has the drawbacks that a time-consuming process is involved of previously crushing ferrite into powder and then uniformly mixing the powder with the adhesive, thus undesirably leading to an increase in the cost of producing the cable 34 and consequently a temperature variation-site detecting apparatus.
Further the ferrite powder contained in the magnetic material 38 rather tends to give rise to the formation of a diamagnetic field, reducing the permeability of the magnetic member 38. Therefore, a decline appears in the changes of the inductance of the cable 32 resulting from temperature variations near the prescribed Curie temperature of the magnetic material 38, and consequently in the sensitivity with which the temperature variation-site detecting apparatus detects temperature variations.